1. Field of the Invention
The present invention relates generally to a mobile communication system supporting an IP multimedia subsystem, and in particular, to a method and apparatus for transmitting and receiving an IMS control message.
2. Description of the Related Art
UMTS (Universal Mobile Telecommunication Service), which is a 3rd generation mobile communication system, WCDMA (Wideband Code Division Multiple Access) based on European mobile communication systems, GSM (Global System for Mobile Communications) and GPRS (General Packet Radio Services), has been developed toward multimedia communications that transmit a large volume of data such as packet data and circuit data beyond voice service.
FIG. 1 schematically illustrates the configuration of a typical mobile communication system, UMTS.
Referring to FIG. 1, the mobile communication system includes a CN (Core Network) 10 and a plurality of RNSs (Radio Network Subsystems) 11 and 12. The plurality of RNSs 11 and 12 form a UTRAN (UMTS Terrestrial Radio Access Network). The CN 10 is comprised of an SGSN and a GGSN to connect the UTRAN to a packet data network such as the Internet.
The RNSs 11 and 12 include RNCs (Radio Network Controllers) 14 and 17 and a plurality of Node Bs 15, 16, 18 and 19. To be more specific, the RNS 11 has the RNC 14 and the Node Bs 15 and 16. The RNS 12 has the RNC 17 and the Node Bs 18 and 19. The RNCs 14 and 17 are classified into a serving RNC, a drift RNC, and a control RNC. The serving RNC manages information about each UE and is responsible for data transmission with the CN 10, the drift RNC wirelessly connects directly to a UE, and the control RNC controls radio resources for each Node B.
The RNCs 14 and 17 are connected to the Node Bs 15, 16, 18 and 19 via an Iub interface. The RNCs 14 and 17 are connected to each other via an Iur interface. While not shown in FIG. 1, a UE 13 is connected to the UTRAN via a Uu interface. The RNCs 14 and 17 allocate radio resources to the Node Bs 15, 16, 18 and 19, and the Node Bs 15, 16, 18 and 19 actually provide radio the allocated resources to the UE 13. Radio resources are configured on a cell basis and radio resources provided by each Node B are those for a specific cell managed by the Node B. The UE 13 establishes a radio channel using radio resources for a specific cell managed by the Node Bs 15, 16, 18 and 19 and transmits/receives data on the established radio channel. Since the UE 13 identifies only a physical channel configured on a cell basis, distinction between a Node B and a cell is meaningless. Therefore, the Node B is interchangeably used with the cell hereinafter.
With reference to FIG. 2, the Uu interface between a UE and an RNC will be described. The Iu, Iub or Uu interface is used for communications between Nodes illustrated in FIG. 2. The Uu interface is defined separately on a control plane (C-plane) for processing control signals and on a user plane (U-plane).
The control plane is comprised of an RRC (Radio Resource Control) layer 20, an RLC (Radio Link Control) layer 21, and an RLC layer 23. The RLC layer 23 has RLC #1 (23a) to RLC #m (23d).
The user plane is comprised of a PDCP (Packet Data Convergency Protocol) layer 21, a BMC (Broadcast/Multicast Control) layer 22, and RLC #1 (23c) to RLC #n (23d).
Data on the control plane and the user plane are provided to a MAC layer 25 on a logical channel 24 and then to a physical layer 27 on a transport channel 26. The PDCP layer 21, the BMC layer 22, and the RLC layer 23 correspond to Layer 2 (L2) and the physical layer 27 corresponds to Layer 1 (L1) in an OSI (Open Systems Interconnection) system model.
In the above mobile communication system, IMS (IP Multimedia Subsystem) refers to a system that provides a traditional circuit-switched service such as voice service in the form of packet service. For wide use of the IMS, similar call quality and call setup delay to those in the traditional circuit-switched service must be ensured. Control messages used for signaling such as call setup in the IMS are called IMS control messages. A call setup procedure is carried out through message exchanges prior to data transmission in a service where two or more users participate like voice service.
FIG. 3 illustrates a circuit-switched call setup procedure in the typical mobile communication system.
Referring to FIG. 3, upon receipt of a call setup message for a particular UE in step 30, the CN pages the UE in step 31. Upon receipt of the PAGING message, the UE establishes an RRC connection with the RNC in step 32. The RRC connection is set up before the UE starts communications in the UMTS network and RRC messages are transmitted/received on the RRC connection. The RRC connection setup amounts to setting up an SRB (Signaling Radio Bearer) for use in transmission/reception of RRC messages.
In step 33, the UE transmits a PAGING RESPONSE message via the SRB to the RNC. The RNC establishes a signaling connection with the CN to transmit the PAGING RESPONSE message in step 34.
The signaling connection is a logical connection for transmission/reception of control messages on the Iu interface for each UE. It is set up by the first NAS (Non Access Stratum) message that the UE transmits after the RRC connection. The NAS message is a control message in the case where a protocol terminal point resides on the CN. MM (Mobility Management), CC (Call Control), and SM (Session Management) messages are NAS messages.
In step 35, the RNC transmits the PAGING RESPONSE message to the CN via the Iu signaling connection. Upon receipt of the PAGING RESPONSE message, the CN determines that the UE is now able to receive the call setup message and transmits to the RNC the call setup message via the Iu signaling connection in step 36. In step 37, the RNC transmits to the UE a call setup response message via the established SRB.
As described above, the circuit-switched mobile communication system exchanges control messages associated with call setup on the control plane involving the Iu signaling connection and the SRB.
In comparison, call setup-associated control messages are exchanged on the user plane in a packet-switched call setup procedure. For this purpose, the user plane must be first established by exchanging control messages, causing an additional delay.
FIG. 4 illustrates a packet-switched call setup procedure in the typical mobile communication system.
Referring to FIG. 4, upon receipt of a call setup message for a particular UE in step 40, the CN pages the UE in step 41. The call setup message can be a SIP (Session Initiation Protocol) INVITE message.
Upon receipt of the PAGING message, the UE establishes an RRC connection with the RNC in step 42 and transmits to the RNC a PAGING RESPONSE message via an SRB established by the RRC connection in step 43. In step 44, the RNC establishes an Iu signaling connection with the CN before transmitting the PAGING RESPONSE message.
In step 45, the RNC transmits the PAGING RESPONSE message to the CN via the Iu signaling connection.
Upon receipt of the PAGING RESPONSE message, the CN establishes a user plane to transmit the call setup message to the UE in step 46.
The user plane setup refers to generation of PDCP/RLC/MAC-layer entities related to message processing on the user plane. When the user plane setup is completed, the CN transmits an INVITE message to the RNC on the user plane in step 47. In step 48, the RNC transmits the call setup message to the UE on the user plane.
The above-described packet-switched call setup procedure third requires the user plane setup. Therefore, a call setup delay is increased, compared to the circuit-switched call setup procedure.